[PATCH] capabilites: allow the application of capability limits tousermode helpers

From: Eric Paris
Date: Tue Mar 29 2011 - 17:01:23 EST

There is no way to limit the capabilities of usermodehelpers. This problem
reared its head recently when someone complained that any user with
cap_net_admin was able to load arbitrary kernel modules, even though the user
didn't have cap_sys_module. The reason is because the actual load is done by
a usermode helper and those always have the full cap set. This patch addes new
sysctls which allow us to bound the permissions of usermode helpers.

/proc/sys/kernel/usermodehelper/bset
/proc/sys/kernel/usermodehelper/inheritable

You must have CAP_SYS_MODULE and CAP_SET_PCAP to change these (changes are
&= ONLY). When the kernel launches a usermodehelper it will do so with these
as the bset and pI.

-v2: make globals static
create spinlock to protect globals

-v3: require both CAP_SET_PCAP and CAP_SYS_MODULE
Signed-off-by: Eric Paris <eparis@xxxxxxxxxx>
No-objection-from: Serge E. Hallyn <serge.hallyn@xxxxxxxxxxxxx>
Acked-by: David Howells <dhowells@xxxxxxxxxx>
---

include/linux/kmod.h | 3 ++
kernel/kmod.c | 100 ++++++++++++++++++++++++++++++++++++++++++++++++++
kernel/sysctl.c | 6 +++
3 files changed, 109 insertions(+), 0 deletions(-)

diff --git a/include/linux/kmod.h b/include/linux/kmod.h
index 6efd7a7..79bb98d 100644
--- a/include/linux/kmod.h
+++ b/include/linux/kmod.h
@@ -24,6 +24,7 @@
#include <linux/errno.h>
#include <linux/compiler.h>
#include <linux/workqueue.h>
+#include <linux/sysctl.h>

#define KMOD_PATH_LEN 256

@@ -109,6 +110,8 @@ call_usermodehelper(char *path, char **argv, char **envp, enum umh_wait wait)
NULL, NULL, NULL);
}

+extern struct ctl_table usermodehelper_table[];
+
extern void usermodehelper_init(void);

extern int usermodehelper_disable(void);
diff --git a/kernel/kmod.c b/kernel/kmod.c
index 9cd0591..3571d1e 100644
--- a/kernel/kmod.c
+++ b/kernel/kmod.c
@@ -25,6 +25,7 @@
#include <linux/kmod.h>
#include <linux/slab.h>
#include <linux/completion.h>
+#include <linux/cred.h>
#include <linux/file.h>
#include <linux/fdtable.h>
#include <linux/workqueue.h>
@@ -43,6 +44,13 @@ extern int max_threads;

static struct workqueue_struct *khelper_wq;

+#define CAP_BSET (void *)1
+#define CAP_PI (void *)2
+
+static kernel_cap_t usermodehelper_bset = CAP_FULL_SET;
+static kernel_cap_t usermodehelper_inheritable = CAP_FULL_SET;
+static DEFINE_SPINLOCK(umh_sysctl_lock);
+
#ifdef CONFIG_MODULES

/*
@@ -132,6 +140,7 @@ EXPORT_SYMBOL(__request_module);
static int ____call_usermodehelper(void *data)
{
struct subprocess_info *sub_info = data;
+ struct cred *new;
int retval;

spin_lock_irq(&current->sighand->siglock);
@@ -153,6 +162,19 @@ static int ____call_usermodehelper(void *data)
goto fail;
}

+ retval = -ENOMEM;
+ new = prepare_kernel_cred(current);
+ if (!new)
+ goto fail;
+
+ spin_lock(&umh_sysctl_lock);
+ new->cap_bset = cap_intersect(usermodehelper_bset, new->cap_bset);
+ new->cap_inheritable = cap_intersect(usermodehelper_inheritable,
+ new->cap_inheritable);
+ spin_unlock(&umh_sysctl_lock);
+
+ commit_creds(new);
+
retval = kernel_execve(sub_info->path,
(const char *const *)sub_info->argv,
(const char *const *)sub_info->envp);
@@ -418,6 +440,84 @@ unlock:
}
EXPORT_SYMBOL(call_usermodehelper_exec);

+static int proc_cap_handler(struct ctl_table *table, int write,
+ void __user *buffer, size_t *lenp, loff_t *ppos)
+{
+ struct ctl_table t;
+ unsigned long cap_array[_KERNEL_CAPABILITY_U32S];
+ kernel_cap_t new_cap;
+ int err, i;
+
+ if (write && (!capable(CAP_SET_PCAP) ||
+ !capable(CAP_SYS_MODULE)))
+ return -EPERM;
+
+ /*
+ * convert from the global kernel_cap_t to the ulong array to print to
+ * userspace if this is a read.
+ */
+ spin_lock(&umh_sysctl_lock);
+ for (i = 0; i < _KERNEL_CAPABILITY_U32S; i++) {
+ if (table->data == CAP_BSET)
+ cap_array[i] = usermodehelper_bset.cap[i];
+ else if (table->data == CAP_PI)
+ cap_array[i] = usermodehelper_inheritable.cap[i];
+ else
+ BUG();
+ }
+ spin_unlock(&umh_sysctl_lock);
+
+ t = *table;
+ t.data = &cap_array;
+
+ /*
+ * actually read or write and array of ulongs from userspace. Remember
+ * these are least significant 32 bits first
+ */
+ err = proc_doulongvec_minmax(&t, write, buffer, lenp, ppos);
+ if (err < 0)
+ return err;
+
+ /*
+ * convert from the sysctl array of ulongs to the kernel_cap_t
+ * internal representation
+ */
+ for (i = 0; i < _KERNEL_CAPABILITY_U32S; i++)
+ new_cap.cap[i] = cap_array[i];
+
+ /*
+ * Drop everything not in the new_cap (but don't add things)
+ */
+ spin_lock(&umh_sysctl_lock);
+ if (write) {
+ if (table->data == CAP_BSET)
+ usermodehelper_bset = cap_intersect(usermodehelper_bset, new_cap);
+ if (table->data == CAP_PI)
+ usermodehelper_inheritable = cap_intersect(usermodehelper_inheritable, new_cap);
+ }
+ spin_unlock(&umh_sysctl_lock);
+
+ return 0;
+}
+
+struct ctl_table usermodehelper_table[] = {
+ {
+ .procname = "bset",
+ .data = CAP_BSET,
+ .maxlen = _KERNEL_CAPABILITY_U32S * sizeof(unsigned long),
+ .mode = 0600,
+ .proc_handler = proc_cap_handler,
+ },
+ {
+ .procname = "inheritable",
+ .data = CAP_PI,
+ .maxlen = _KERNEL_CAPABILITY_U32S * sizeof(unsigned long),
+ .mode = 0600,
+ .proc_handler = proc_cap_handler,
+ },
+ { }
+};
+
void __init usermodehelper_init(void)
{
khelper_wq = create_singlethread_workqueue("khelper");
diff --git a/kernel/sysctl.c b/kernel/sysctl.c
index c0bb324..965134b 100644
--- a/kernel/sysctl.c
+++ b/kernel/sysctl.c
@@ -56,6 +56,7 @@
#include <linux/kprobes.h>
#include <linux/pipe_fs_i.h>
#include <linux/oom.h>
+#include <linux/kmod.h>

#include <asm/uaccess.h>
#include <asm/processor.h>
@@ -616,6 +617,11 @@ static struct ctl_table kern_table[] = {
.child = random_table,
},
{
+ .procname = "usermodehelper",
+ .mode = 0555,
+ .child = usermodehelper_table,
+ },
+ {
.procname = "overflowuid",
.data = &overflowuid,
.maxlen = sizeof(int),

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